Skip a Letter:

If your finger doesn't come down with enough force, a letter may not get typed. This can happen more with the pinky or on the bottom row.

Double a Letter:

In typing several letters in close proximity, a key can be accidentally re-tapped.

Transpose Letters:

Some say that typing t-e-h for t-h-e is the most common spelling error. This suggests that perhaps the messages to the fingers are not keeping up with the keyboarder's thinking ahead to the next word.

Skip a Space:

This can happen if the space bar isn't hit firmly enough or if it needs to be hit with the non dominant hand which is less accustomed to tapping it. In addition, the appearance of a missed space can come about due to a transposition of the space with a letter - often the final letter of a word that gets tagged onto the front of the next word by the space/letter transposition. An example - aboutit instead of about it.

Miss a Key:

This can happen both when the keyboardist hits a neighboring key instead of the key intended and also with the keys that require a reach or that are less frequently used. So, one may mistype b and numbers like 1, 6, and 7 as well as the `, ~, ^, &, =, and +. In addition, when typing a key combination, the modifying key may be missed, resulting in spellings like aren;t instead of aren't.

Insert a Key:

When the fingers are flying, an extra letter that doesn't belong in the word may get introduced.